1. Field of the Invention
The present invention relates to an image reader and an image reading method.
2. Description of the Related Art
In some cases, an image reader temporarily stops reading an original document during reading operation by moving a carriage or the original document. This discontinuous operation is referred to as intermittent operation hereinafter. Examples of the factors inducing intermittent operation include electrical noises and a case where image is not normally read due to any cause (if reading continues after the occurrence of the case, image quality deteriorates). When such intermittent operation factor occurs, moving operation of the carriage or the original document needs to temporarily stop until the intermittent operation factor is eliminated. If the moving operation of the carriage or the original document does not stop, discontinuity occurs between image data before the temporary stop of reading and image data after the resuming of reading.
The carriage or the original document cannot instantly stop from a steady speed (a moving speed thereof when image of the original document is read) or instantly reach the steady speed from the stopped state because the moving speed of the carriage or the original document is governed by the law of inertia. In other words, the carriage or the original document needs to stop after deceleration from the steady speed or to reach the steady speed after acceleration from the stopped state. If image of the original document is read during deceleration or acceleration, abnormality such as distortion occurs in read image data because the moving speed of the carriage or the original document differs from that in reading at the steady speed.
The following methods have been known as those for preventing such problem. A first method is as follows: when reading of image of an original document is temporarily stopped in flatbed reading or automatic document feeder (ADF) reading, a carriage or the original document is returned from a position at which reading is stopped (a reading stop position) by a distance necessary for acceleration when reading resumes, and stopped. Reading of image restarts when the carriage or the original document reaches the reading stop position. For example, refer to Japanese Patent Application Laid-open No. 2001-024859. This method can prevent the occurrence of abnormality caused by intermittent operation because no discontinuity occurs between image data before the temporary stop of reading and image data after the resuming of reading, and also image of the original document is read only at a stable moving speed of the carriage or the original document. A second method is based on a point of view that the ADF generally cannot feed an original document in a returning direction, and as follows: when reading of image of an original document is temporarily stopped in ADF reading, a carriage is moved in an original document feed direction by a distance corresponding to an overrun of the original document after reading is stopped, and stopped. Reading operation of the original document resumes from the stopped position. For example, refer to Japanese Patent No. 3701621. This method can also prevent the occurrence of abnormality caused by intermittent operation because no discontinuity occurs between image data before the temporary stop of reading and image data after the restarting of reading, and image of the original document is read only at a stable moving speed of the original document.
The related art, however, has a problem in that operation is cumbersome. When reading of image of an original document is temporarily stopped due to the occurrence of an intermittent operation factor, it is required that the movement of a carriage or the original document needs to be stopped every occurrence of the intermittent operation factor, and the carriage or the original document is moved in the returning direction, or the carriage is moved in an original document feed direction in ADF reading in which the original document cannot be fed in the returning direction.
The applicant of the present invention proposed an image reader and an image reading method in Japanese Patent Application No. 2008-236878 (Japanese Patent Application Laid-open No. 2010-074289) to address the problems of the related art. The image reader and the image reading method employ first and second line sensors that read image of an original document at positions located physically apart from each other in a sub-scanning direction. The use of the two line sensors enables image data of the original document to be made continuous before the temporary stop of reading and after the resuming of reading when reading of the original document is temporarily stopped due to the occurrence of an intermittent operation factor, without stopping the movement of the carriage or the original document every occurrence of the intermittent operation factor as in related art. Japanese Patent Application No. 2008-236878, however, newly arises problems in that the structure becomes complex, thereby increasing costs because the use of the first and second line sensors for reading image of the original document requires analog front ends (AFE) that digital-convert analog image signals and other processing units that carry out necessary correction processing for the two line sensors, for example.